When relatively large quantities of gases, such as oxygen, nitrogen, argon or hydrogen, are required by a use point, the gases are generally delivered in liquid form to a storage tank near the use point. The liquefied gas is then vaporized and passed on as needed to the use point.
The gas must be delivered to the use point at a pressure specified by the use point requirements. However, for safety reasons, liquefied gases cannot be transported over public roads from a production plant to the liquid storage tank near the use point at pressures significantly above atmospheric. For most gases the use point pressure requirement is met by pumping the liquefied gas from the transport vehicle into the storage tank using a liquid pump to increase its pressure. The liquefied gas is stored in the storage tank at this high pressure and, upon demand from the use point, is vaporized at the high pressure and delivered to the use point as pressurized gas meeting the use point pressure requirements.
This pressurizing procedure may be used effectively with all liquefied gases except for helium. Because of its unusual physical properties, it is not practical to pump liquid helium to a significantly higher pressure. Because of the very low heat of vaporization of liquid helium, the heat introduced to the liquid by the action of the liquid pump causes a significant amount of the liquid to be vaporized and thus lost. Furthermore, because the density of cold helium gas is not much different from that of liquid helium, every time a storage tank is filled with liquid helium, a large amount of the cold helium gas within the tank is displaced and lost; at higher pressures these displacement losses are even higher. Accordingly, heretofore, helium has been delivered to use points by cylinder or tube trailer as high pressure gas.
While this helium delivery system is satisfactory for most uses of helium, it presents a problem when the use point requires gaseous helium of ultra high purity. This is because the pumping activity required to achieve the requisite pressure invariably causes some impurity contamination of the gaseous helium. Heretofore the highest purity gaseous helium generally available has had an impurity concentration of about 30 to 50 parts per million (ppm). Ultra high purity helium gas is being increasingly required by, for example, the electronics industry.
Therefore, it is an object of this invention to provide a method to deliver efficiently ultra high purity helium gas to a use point at the use point pressure requirement.